The long-term goals of this proposal are to understand the precise mechanisms regulating cell- surface level, localization and targeting of cardiovascular ion channels. Atrial fibrillation is the most common cardiac arrhythmia affecting more than 2 million Americans and results in significant mortality due to stroke and heart failure. This electrical instability in the human heart can occur through an acquired disorder attributable to ion channel remodeling secondary to structural heart disease or as a result from a primary genetic defect in ion channel function. Kv1.5 is a prominent cardiovascular K+ channel that is vital for atrial repolarization in the human heart. Alterations in the cell surface expression of functional Kv1.5 contribute to the pathophysiology of paroxysmal and persistent atrial fibrillation as well as chronic hypoxic pulmonary hypertension. Remarkably, despite the clear links between changes in Kv1.5 surface expression and cardiovascular disease, relatively little is known regarding the mechanisms controlling its plasma membrane targeting or localization. Recently, we have discovered an unexpected dynamic trafficking of Kv1.5 at the myocyte plasma membrane and demonstrated a role for internalization and recycling in the maintenance of steady- state ion channel surface levels. Nonetheless, similar to most proteins in the heart, the molecular machinery and the regulatory mechanisms controlling the surface levels of Kv1.5 in atrial myocytes remain unclear. We hypothesize that, in atrial myocytes, Kv1.5 surface levels are controlled by the coordinated movement of kinesin and myosin motors coupled to the channel by Rab GTPases that act to regulate channel internalization and recycling. Moreover, we propose that this process is modulated by cholinergic stimulation and can be therapeutically controlled by antiarrhythmic drug binding. Therefore in Specific Aim 1, we will define the molecular machinery involved in internalization and recycling of Kv1.5 in mouse and human atrial myocytes. In Specific Aim 2, we will determine the contribution and mechanisms of cholinergic and adrenergic stimulation to the direct modulation of Kv1.5 internalization and recycling. In Specific Aim 3, we will determine the mechanisms of antiarrhythmic drug-induced Kv1.5 channel internalization. Successful completion of our proposed studies will undoubtedly contribute to our knowledge of the events underlying the pathophysiological conditions characterized by altered Kv1.5 surface expression and likely provide novel insight into novel therapeutic strategies designed to manipulate specific ion channel trafficking pathways for the treatment of cardiovascular channelopathies and modulation of cardiac electrical excitability.